Embodiments of the invention relates to a process for the preparation of copper containing molecular sieves with the CHA structure having a silica to alumina mole ratio greater than about 10. In specific embodiments, copper exchange is conducted via wet state exchange and prior to coating.
Both synthetic and natural zeolites and their use in promoting certain reactions, including the selective reduction of nitrogen oxides with a reductant like ammonia, urea or a hydrocarbon in the presence of oxygen, are well known in the art. Zeolites are aluminosilicate crystalline materials having rather uniform pore sizes which, depending upon the type of zeolite and the type and amount of cations included in the zeolite lattice, range from about 3 to 10 Angstroms in diameter. Chabazite (CHA) is a small pore zeolite with 8 member-ring pore openings (˜3.8 Angstroms) accessible through its 3-dimensional porosity. A cage like structure results from the connection of double six-ring building units by 4 rings.
X-ray diffraction studies on cation locations in Chabazite have identified seven cation sites coordinating with framework oxygens, labeled A, B, C, D, F, H, and I. They are located in the center of double six-membered ring, on or near the center of the six-membered ring in Chabazite cage, and around the eight-membered ring of the chabazite cage, respectively. The C site is located slightly above the six-membered ring in the Chabazite cage and the F, H and I sites are located around the eight-membered ring in the Chabazite cage (see Mortier, W. J. “Compilation of Extra Framework Sites in Zeolites”, Butterworth Scientific Limited, 1982, p 11 and Pluth, J. J., Smith, J. V., Mortier, W. J., Mat. Res. Bull., 12 (1977) 1001).
The catalysts employed in the SCR process ideally should be able to retain good catalytic activity over the wide range of temperature conditions of use, for example, 200° C. to 600° C. or higher, under hydrothermal conditions. Hydrothermal conditions are often encountered in practice, such as during the regeneration of a soot filter, a component of the exhaust gas treatment system used for the removal of particles.
Metal-promoted zeolite catalysts including, among others, iron-promoted and copper-promoted zeolite catalysts, for the selective catalytic reduction of nitrogen oxides with ammonia are known. Iron-promoted zeolite beta (U.S. Pat. No. 4,961,917) has been an effective commercial catalyst for the selective reduction of nitrogen oxides with ammonia. Unfortunately, it has been found that under harsh hydrothermal conditions, for example exhibited during the regeneration of a soot filter with temperatures locally exceeding 700° C., the activity of many metal-promoted zeolites begins to decline. This decline is often attributed to dealumination of the zeolite and the consequent loss of metal-containing active centers within the zeolite.
WO 2008/106519 discloses a catalyst comprising: a zeolite having the CHA crystal structure and a mole ratio of silica to alumina greater than 15 and an atomic ratio of copper to aluminum exceeding 0.25. The catalyst is prepared via copper exchanging NH4+-form CHA with copper sulfate or copper acetate. The copper concentration of the aqueous copper sulfate ion-exchange step varies from 0.025 to 1 molar, where multiple copper ion-exchange steps are needed to attain target copper loadings. The copper concentration of the aqueous copper acetate ion-exchange step varies from 0.3 to 0.4 molar conducting a wet state exchange, thus, a separate copper exchange step prior to the coating process. Conducting copper exchange step during the coating the copper concentration is 0.25 and 0.12 (examples 16 and 17).
US 2008/0241060 and WO 2008/132452 disclose that zeolite material can be loaded with iron and/or copper. In the examples of US 2008/0241060 the copper ion-exchange is not described. It is stated in WO 2008/132452 that multiple aqueous ion-exchanges were carried out to target 3 wt % Cu. No details of reaction conditions were provided.
Dedecek et al. describes in Microporous and Mesoporous Materials 32 (1999) 63-74 a direct copper exchange into Na+-, Ca2+-, Cs+-, Ba2+-form of Chabazite. An aqueous solution of copper acetate is used with copper concentrations varying between 0.20 and 7.6 wt %, thus between 0.001 and 0.1 molar. The liquid to solid ratio varies between 20 and 110. The silica to alumina ratio is between 5 and 8.
Although typically the catalysts are produced in a similar manner, the NOx conversion activity of the catalysts fluctuates strongly from one experiment to the other (see table 1). The synthesis of copper containing molecular sieve with the CHA structure is a remarkable complex reaction. In general, the preparation includes four main sub-steps i) crystallization of the organic template containing Na/K-Chabazite, ii) calcination of Na/K-Chabazite, iii) NH4-exchange to form NH4-Chabazite and iv) metal-exchange into to NH4-Chabazite to form metal-Chabazite.
In addition, all these sub-steps like metal-exchange into to NH4-Chabazite to form metal-Chabazite can be divided in additional sub-sub-steps a) forming metal-Chabazite step, b) separation step, c) optionally drying step and d) calcination step. For example the sub-sub step a), the forming of metal-containing zeolite may be influenced by (1) the starting material chosen, (2) concentration of each starting material, (3) liquid:solid ratio of that starting material, (4) reaction time, (5) reaction temperature, (6) pH value or (7) additional reaction conditions like stirring or so.
Up to now the most important process features that cause fluctuations in the NOx conversion activity are not determined yet.
In general, the SCR catalyst based on that Chabazite molecular sieve should exhibit comparable NOx conversion activity with the catalysts of the state of the art obtained via multi-step synthesis (copper exchange into NH4-Chabazite). In general, the catalyst should exhibit both good low temperature NOx conversion activity (NOx conversion>50% at 200° C.) and good high temperature NOx conversion activity (NOx conversion>70% at 450° C.). The NOx activity is measured under steady state conditions at maximum NH3-slip conditions in a gas mixture of 500 ppm NO, 500 ppm NH3, 10% O2, 5% H2O, balance N2 at a volume-based space velocity of 80,000 h−1.
There is an on-going desire to improve the process of preparing copper containing molecular sieves with the CHA structure.